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He said he made the suggestion to Australian Prime Minister Julia Gillard last night when the pair had been texting each other.
They were working out what they could put up as prizes for a winning bet.
"It's got to be something useful," he said. "It's hard to swap ties when one side doesn't wear a tie. I suggested a coalmine actually - they've got a few in Australia.
"We can give her the pavlova recipe."
"Anyway we're working it out," he said before before heading into National's weekly caucus meeting.
Australia's mining boom has helped to keep it out of recession during the global financial crisis and Mr Key was likely referring to that.
But the mention of coalmines in New Zealand at present is synonymous with tragedy with the Pike River mining disaster being relived at the hearing of the Royal Commission of Inquiry in Greymouth.